Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to 1.9.0, Authlib's RFC 9101 JWTAuthenticationRequest passes the unauthenticated, client-controlled request_uri parameter to the abstract get_request_object() hook before applying a destination or registration policy; the hook itself performs no network request, while the documentation demonstrates an unrestricted HTTP fetch. In deployments whose integrator implements that hook as an unrestricted outbound fetch, a remote attacker who knows a valid public client identifier can make the authorization server request internal, link-local, or otherwise unintended URLs and can distinguish fetch outcomes through different error responses and timing. Deployments whose custom hook already applies a secure allowlist, destination controls, and retrieval limits are not affected. The JWTAuthenticationRequest implementation in authlib/oauth2/rfc9101/authorization_server.py forwards request_uri to get_request_object() before an application-approved retrieval policy accepts the destination. This issue is fixed in version 1.9.0.

Affected: PyPI: authlib

### Summary

Authlib's RFC 9101 (JAR) extension passes the attacker-controlled `request_uri` authorization parameter straight to the `get_request_object()` fetch hook with no validation, at a pre-authentication endpoint. Authlib already defines the pre-registration allowlist for this (`request_uris` client metadata and `require_request_uri_registration` server metadata) but the extension never consults it, and both docstrings instruct integrators to implement the hook as an unrestricted `requests.get(request_uri)`. Deployments following the documented pattern get an unauthenticated SSRF primitive. ### Details

Verified against authlib 1.7.2 and current `main`. All line numbers below are from `authlib/oauth2/rfc9101/authorization_server.py`.

The extension registers itself on the `before_get_authorization_grant` hook (lines 62-65), so it runs on every authorization request before any user or client authentication. `support_request_uri` defaults to `True` (line 58).

The only gate before the sink is that `client_id` names an existing client (lines 91-103). A `client_id` is a public, non-secret identifier.

The sink at line 159 forwards the raw parameter with no scheme, host, or allowlist check:

```python def _get_raw_request_object(self, request: OAuth2Request) -> str: if "request_uri" in request.payload.data: raw_request_object = self.get_request_object( request.payload.data["request_uri"] ) if not raw_request_object: rais…

Affected: Ubuntu:Pro:22.04:LTS: python-authlib, Ubuntu:Pro:24.04:LTS: python-authlib, Ubuntu:26.10: python-authlib, Ubuntu:Pro:26.04:LTS: python-authlib

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to 1.9.0, Authlib's RFC 9101 JWTAuthenticationRequest passes the unauthenticated, client-controlled request_uri parameter to the abstract get_request_object() hook before applying a destination or registration policy; the hook itself performs no network request, while the documentation demonstrates an unrestricted HTTP fetch. In deployments whose integrator implements that hook as an unrestricted outbound fetch, a remote attacker who knows a valid public client identifier can make the authorization server request internal, link-local, or otherwise unintended URLs and can distinguish fetch outcomes through different error responses and timing. Deployments whose custom hook already applies a secure allowlist, destination controls, and retrieval limits are not affected. The JWTAuthenticationRequest implementation in authlib/oauth2/rfc9101/authorization_server.py forwards request_uri to get_request_object() before an application-approved retrieval policy accepts the destination. This issue is fixed in version 1.9.0.